1. Field of the Invention
The present invention relates to a secondary battery, and in particular, to a secondary battery which improves the structure of non-active material portions of the electrode assembly to enhance the current collecting efficiency.
2. Description of the Related Art
Secondary batteries are classified into low-capacity secondary batteries (referred to hereinafter as “low-capacity batteries”), which use one or more battery cells packaged in the form of a pack, and motor driving high-capacity secondary batteries (referred to hereinafter as “high-capacity batteries”), which use tens of battery cells packaged into a battery pack.
The low-capacity batteries are used as the power supply for small electronic devices, such as cellular telephones, notebook computers, and camcorders, while the high-capacity batteries are used as the power supply for driving motors in hybrid electric vehicles and the like.
When the low-capacity battery is constructed with a cell, it is mainly formed with a cylindrical shape or a prismatic shape. An insulating separator is disposed between band-typed positive and negative electrodes, which are wound in a spiral shape to form an electrode assembly. The electrode assembly is inserted into a case to construct a battery.
Conductive tabs are attached to the positive and the negative electrodes as lead terminals for collecting the electric current respectively generated by the positive and the negative electrodes. The conductive tabs are welded to the positive and the negative electrodes, and transfer the electric current generated by the positive and the negative electrodes to positive and the negative electrode terminals.
When the above-described low-capacity battery structure is directly applied for use in a high-capacity battery, it cannot satisfy the operational characteristics of the latter with respect to capacity or power. Therefore, it has been proposed that a multi-tap structure should be introduced to increase the battery capacity or power.
The multi tap structure is structured such that a plurality of taps are connected to the positive and the negative electrodes of the electrode assembly, and merged into positive and negative electrode taps, which are then connected to the positive and the negative electrode terminals.
Moreover, a current collecting plate structure can replace the tap structure to collect the electric current from the electrode assembly in the high-capacity battery.
The current collecting plate structure transfers the electric current from the electrode assembly to the positive and the negative electrode terminals by connecting the current collecting plate to the positive and the negative electrodes of the electrode assembly. Such a structure has an advantage in that it enlarges the contact area between the current collecting plate functioning as a lead element and the positive and negative electrodes of the electrode assembly, as compared to the tap structure.
For this reason, with the secondary batteries forming a motor driving secondary battery module, the current collecting uses the current collecting plate.
When the motor driving secondary battery is formed with a prismatic shape rather than with a cylindrical shape, positive and negative electrode terminals are provided on a cap plate fitted to the case. The electrode terminals are electrically connected to the positive and the negative electrodes of the electrode assembly contained within the case to collect the electric current generated by the positive and the negative electrodes.
The structure of the electrical connections between the electrode terminals and the electrode assembly directly influences the current collecting efficiency of the prismatic-shaped battery. Accordingly, it is necessary to make the electrical connection in a reasonable manner.
However, when the lead elements are tightly adhered to the contact area of the electrode assembly contacting the electrode terminals to electrically interconnect the electrode terminals and the electrode assembly, a gap occurs at that contact area, making it difficult to achieve excellent connections.
This phenomenon occurs due to the shape of the non-active material portions of the positive and negative electrodes of the electrode assembly, to which the lead elements are tightly adhered.